The Yellowstone hotspot in space and time: Nd and Hf isotopes in silicic magmas

Over the course of its 16 m.y. history, the Yellowstone hotspot has produced silicic magmas exhibiting systematic, and often sympathetic, variations in isotopic and chemical composition, temperature and frequency of eruption. Nd and Hf isotopic ratios vary systematically from initial eruptions at ∼ 16 Ma, contemporaneous with basaltic volcanism in eastern Oregon and Washington, to the present day Yellowstone Volcanic Plateau. Nd and Hf isotopic ratios co-vary and span the range of most terrestrial samples, reflecting mixing of mantle and crustal sources. Earliest erupted silicic magmas were hot (in excess of 1050 °C), relatively less evolved and have isotopic ratios within the range of contemporaneous Columbia River flood basalts. The transit of the hotspot across the lithospheric boundary between the western accreted oceanic terrain and the Precambrian craton at 15 Ma is marked by shifts in εNd from + 4 to − 11 and in εHf from + 10 to − 10. The duration of the transit yields a crustal magma source diameter of ∼ 70 km. In the interval from 14 to 9 Ma, εNd systematically increases from − 11 to − 7, recording a minimum increase in the mantle component from 5% to 30%. The mantle component could be twice as great, depending upon the isotopic composition of crust and mantle reservoirs. In this same interval, peak temperatures of ∼ 1000 °C occurred at 9 Ma. The last 8 m.y. are characterized by less frequent eruption of lower temperature (830–900 °C) and more compositionally evolved magmas.